The present invention relates generally to disc drive data storage systems. More particularly, the present invention relates to compensation for errors in servo systems.
Disc drives read and write information along concentric tracks formed on discs. To locate the read-write head on a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo subsystem to position a head over a particular track. The servo fields are written onto the disc when the disc drive is manufactured and are thereafter simply read by the read-write head of the disc drive to determine position. A multi-rate servo system samples the position of the head relative to a particular track at a particular sampling rate and adjusts the position of the head at a rate that is a multiple of the sampling rate by estimating the position of the head between the measured position samples.
It is desired that the head moves along a perfectly circular path around the disk. However, two types of errors prevent heads from following this ideal path. The first type of error is a written-in error that arises during the creation of the servo fields in servo writing process. Written-in errors occur because the write head used to produce the servo fields does not always follow a perfectly circular path mainly due to unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, from disk fluttering, from spindle motor vibration, from disk bending, and from vibrations in the gimbal used to support the head. Because of these disturbances, the track written on the disc is not a perfectly circular path and a head in a disc will then follow a non-circular path.
The second type of error that prevents circular paths is known as track following disturbances. Track following errors arise when a head attempts to follow the path defined by the servo fields. The disturbances can be caused by the same aerodynamic and vibrational effects that create written-in errors. In addition, track following errors can arise because the servo system bandwidth is limited and the head is unable to respond fast enough to high frequency changes in the path defined by the servo fields.
Here, we focus on written-in errors. Written-in errors are often referred to as repeatable runout errors because they cause the same errors at the same location each time the head passes along a track. As track densities increase, these repeatable runout errors begin to limit the track pitch. Specifically, variations between the ideal track path and the actual track path created by the servo fields can result in an inner track path that interferes with an outer track path. This is especially acute when a first written-in error causes a head to be outside of an inner track""s ideal circular path and a second written-in error causes the head to be inside of an outer track""s ideal circular path. This is often referred as track squeeze problem. To avoid limitations on the track pitch, a system is needed to compensate for these repeatable runout errors.
The written-in errors can be compensated by injecting stored compensation values into the servo loop. However, the determination of the compensation values has required complex computations which have often been difficult to implement in disc storage systems. The present invention provides a simple solution to this and other problems and offers other advantages over the prior art.
The present invention relates to compensation for written-in repeatable-runout in disc drives. The present invention relates to disc drives which have runout compensation which solves the above-mentioned problem.
In accordance with one embodiment of the invention, repeatable-runout compensation is provide in a disc storage system in which servo position values are read from a disc surface which indicates the head position relative to a track on the disc surface. Subsequently, compensation values are retrieved from a table of compensation values and a servo position value is compensated based upon the retrieved compensation value.
In accordance with another embodiment of the invention, a disc drive includes a servo loop for positioning a head over a disc. The disc is adapted to store data on a track and the track includes servo fields adapted to store servo information used to indicate position. The head is adapted to sense the servo information located on the disc and to produce a servo position signal therefrom. The servo position signal is combined with the reference signal to produce a position error signal indicative of a difference between an actual and a desired position of the head relative to the track. A servo controller is adapted to generate a servo control signal in response to the position error signal. An actuator, coupled to the servo control, is adapted to move the head in response to the servo control signal. A compensation table, stored in memory or servo field, is adapted to provide a written in repeatable runout compensation value to the servo control signal.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.